Voyage to the bottom of the Puerto Rico Trench: Tales from a free fall current meter

Near-bottom (~2 m) current velocities in the Puerto Rico Trench (~8350 m depth) were measured at 1 Hz for 75 min by a Nortek Deep Water Aquadopp acoustic-Doppler current meter at 19.75° N, 66.40° W, via untethered free descent/ascent vehicle. The April 2008 deployment also recorded 3-axis velocity, temperature, pressure, and instrument heading, pitch, roll, and signal-strength during the 153 min free descent, and while on bottom. Signal strength was above the noise floor for the entire data set, and SNR and velocity standard deviation were within known acceptable bounds above 7000 m. Instrument heading showed a continuous anticlockwise rotation during descent. Doppler vertical velocity during descent is compared to the pressure time derivative. Integration of horizontal velocity during descent suggests a lateral displacement of less than 30 m over the 8.35 km free-fall. Measurements made at impact indicate full functionality of the instrument at depth. Maximum horizontal velocities while on-bottom varied between 1 cm/s and 5 cm/s and were directed roughly along trench axis westward.